BOSTON — For the nation’s oldest fire department, the alarm sounds 234 times a day. Car accidents, medical calls, rescues and fires keep Boston firefighters busy round-the-clock. But while they are equipped with state-of-the-art apparatus and protective clothing, what’s killing them is a danger they often can’t see: cancer. Boston Fire Commissioner Joseph Finn called it an “epidemic.” “We're seeing a lot of younger members in their 40s, early 40s, who've got 20 years on the job, who are developing these cancers at a very young age,” Finn told NBC News. Each month, another three active or just-retired firefighters are diagnosed with cancer. The cancer rate among firefighters is more than twice the rate for Boston residents — and it’s illegal for firefighters in this city to smoke. At the Dana Farber Cancer Center at Brigham and Women’s Hospital, firefighter Glenn Preston is being treated for blood cancer. Firefighter Glenn Preston is being treated for blood cancer at the Dana Farber Cancer Center at Brigham and Women's Hospital. Glenn Preston He’s already had chemotherapy and a bone marrow transplant. He invited NBC News to his hospital room to talk, but the crew had to wear surgical masks and gloves, swabbing the camera gear down with alcohol to lessen the risk of introducing a virus that could prove fatal to Glenn. “It's in the lining of my heart. The tumor's in the lining of my heart now,” he said. Married with four children, Preston is just 41 years old and a native Bostonian. “For me, it's a passion,” he explained. “Other than God, family, and my country. There's nothing I love more than being a Boston firefighter.” In 2002, Preston was among 200 firefighters who responded to a massive inferno at a power plant on the city’s south side. Inside the building, he became separated from his crew as chemicals rained down from the roof, coating his protective turnout gear in a petroleum-jelly-like goo. “That’s the most scared I've ever been in my life, I think.” Boston firefighter Glenn Preston on the job. Picasa / Glenn Preston When he finally made it out, his jacket was covered in a slick slime, possibly containing PCBs. Of the 200 firefighters who responded, a